Pressure sensors may be embedded into various mobile devices such as cell-phones, laptops, tablet computers, and vehicle navigation systems. Mobile devices with the pressure sensors are capable of performing 3-dimension navigation. In the future, applications of 3-dimension navigation may include indoor navigation, and outdoor navigation for tunnel, bridge and overpass. Thus, the navigation system requires an update from Global Positioning System (GPS) with 9-axis inertial sensors (including an accelerometer, a gyroscope and a magnetometer) to GPS with 10-axis inertial navigator (including an accelerometer, a gyroscope, a magnetometer and an air pressure sensor (also called a “barometer”).
Generally, the higher the altitude, the lower the air pressure. The barometer (one kind of pressure sensor) is used to detect the change in air pressure so as to evaluate the change in altitude. A conventional air pressure sensor 1 shown in FIG. 1 includes a substrate 2, a stationary electrode 3 and a cover 4. The cover 4 is disposed on the substrate 2 and shelters the stationary electrode 3. In addition, the cover 4 includes a diaphragm 4-1. A hermetic space 5 is defined between the diaphragm 4-1 and the stationary electrode 3.
The conventional air-pressure sensor 1 includes two types of sensors. One type of the air-pressure sensors is utilized to detect absolute pressure while the other type of the air-pressure sensors is used to measure relative pressure. For detecting the absolute pressure, the hermetic space 5 of the former sensor is vacuum. When such a sensor is placed in an environment where an external air pressure P exists relative to the cover 4, the diaphragm 4-1 will deform. The deformation will change the capacitance between the diaphragm 4-1 and the stationary electrode 3 due to a change in the distance therebetween. In accordance with the change in capacitance, a processor may calculate an air-pressure value. Thus, the air-pressure value is an absolute pressure value of the external air-pressure P.
Regarding the sensor for measuring relative pressure, there is a gas-pressure P1 in the hermetic space 5. As shown in FIG. 2, in a certain altitude, when the external air-pressure maintains at P2, the diaphragm 4-2 will produce deformation d1. The deformation will change the capacitance between the diaphragm 4-2 and the stationary electrode 3-1 due to a change in the distance therebetween. In accordance with the change in capacitance, a processor may calculate an air-pressure value. Thus, the air-pressure value is a relative pressure value between the gas-pressure P1 and the external air-pressure. Either the sensors for absolute pressure or the sensors for relative pressure are capable of calculating the change of altitude according to the above-mentioned pressure values, but in those sensors only a single capacitor is provided, which is liable to error in pressure measurement due to undesirable noise of the single capacitor.
In order to improve the conventional pressure sensors, as shown in FIG. 3, a differential pressure sensor 6 is designed with a sensing capacitor 7 including a movable electrode 7-1 and a stationary electrode 7-2, and with a reference capacitor 8 including two stationary electrodes 8-1 and 8-2. The differential pressure sensor 6 is capable of calculating the differential capacitance between the sensing capacitor 7 and the reference capacitor 8 so as to reduce noise.
FIG. 4 is a top view of the differential pressure sensor 6. As shown in FIG. 4, a reference capacitor 8 of the differential pressure sensor 6 increases the entire area of the differential pressure sensor 6. Thus, it is difficult for the differential pressure sensor 6 to use in a mobile device. FIG. 3 illustrates a cross-sectional view of the differential pressure sensor 6, taken from the line 1-1 in FIG. 4.
A differential pressure sensor 9 shown in FIG. 5 includes a first electrode 9-1, a second electrode 9-2 and a third electrode 9-3. The first electrode 9-1 and the third electrode 9-3 form a reference capacitor while the first electrode 9-1 and the second electrode 9-2 form a sensing capacitor. When an external pressure causes the first electrode 9-1 to deform, the distance between the first electrode 9-1 and the third electrode 9-3 is changed and the capacitance of a stationary electrode is affected. Consequently, an error results in the calculation based on the capacitance difference between the sensing capacitor and the reference capacitor.